In traditional multi-camera production systems, the cameras, base stations and video mixer plus digital effects are closely linked together in a system with an overall low latency.
A simplified example of such a production system is given in FIG. 1.
At the camera location 2, a person with a camera 4 (camera person) frames the picture and keeps it in focus. The camera person works with the camera 4 on the basis of a picture observed in the camera viewfinder (not shown). He generally gets his commands from the director.
In the control area 6 (generally a control room), a camera control operator uses camera controls 8 to control parameters of the camera 4 and its video processing circuits 12, like iris, black, painting and contours, in order to define the “looks” of the picture and to maintain proper video levels and white balance. The camera control operator works on the basis of a picture on a reference monitor 10 at the control location (control room 6).
As the camera location 2 and control area 6 are conventionally close to each other and simply connected by electrical wires 16, 18 (e.g. triax cables), there is no noticeable latency in the communication channel consisting of these wires 16, 18 between the camera 4 and its processing circuits 12 on the one hand, and the control area 6 with camera controls 8 on the other hand. Although the processing circuits 12 are represented on FIG. 1 separate from the camera 4 for the sake of clarity, they are most generally part of the camera 4.
When the camera control operator modifies a parameter on camera controls 8, the information is transmitted via the control electrical wire 18 to the processing circuits 12 of the camera 4. There, the change in value of the parameter becomes effective, thus altering the effects of the processing circuits 12 on the raw (video and audio) signal 20. The change is then visible on the processed signal 22, which is transmitted to the control area 6 via the signal electrical wire 16. At this time, the change is visible on the monitor 10.
This closed control loop needs very low latency (less than 0.1 second) to avoid instability and permit fast control. This is easy to obtain with the conventional structure as described above where the camera location 2 and the control area 6 are close to each other, or connected with a low-latency connection.
The processed signal 22 is ready to be broadcast or transmitted as schematically represented in FIG. 1, for instance via an emitter 14.
The production chain may involve additional steps: for instance, a production control operator may switch between pictures of several cameras and add effects if necessary, on the basis of commands he gets from the director.
In recent years, a lot of new communication channels have been developed, as for instance satellite links and public networks like the Internet, which allow the transmission of information from a wide range of locations on Earth.
It would be very convenient if these new communications channels could be used to transmit video and audio information from the camera to the control area of the broadcaster as it would greatly widen the possibilities for the camera and the camera person to move freely. Furthermore, it could reduce production cost as only part of the production team will have to travel to and stay on the location of the shooting.
The problem with such long distance communication channels is that they suffer noticeable latency which make them totally maladapted to be used as a communication channel between camera and control area. Effectively, if such a communication channel were used in the conventional system described above, the time constant of the closed control loop would be too large to avoid instability and to allow fast control.
This problem is even more important as it is generally desirable to compress the video and audio information transmitted via such communication channels, which further lengthen the latency time of the communication.
The solution of avoiding long distance in the closed control loop by placing the camera controls at the camera location is not totally satisfactory in professional production, as the director wants to entirely monitor the looks or appearance of the picture at the control room (central studio).